From the viewpoint of low transmission loss, silica based glass fibers have been used as optical fibers for long-distance transmission. In order to keep transmission and strength characteristics, employed is a coated optical fiber in which a primary coating made of a relatively soft resin and a secondary coating made of a relatively hard resin are applied to the surface of a glass fiber.
Conventionally used is a coated optical fiber in which, at the same time when a glass fiber is drawn from an optical fiber preform, a primary coating resin is applied thereto and cured thereon so as to form a primary coating, and then a secondary coating resin is applied to and cured on the outer periphery of the primary coating so as to form a secondary coating. On the other hand, recently has been studied from the viewpoint of improving productivity is a method in which both primary and secondary coating resins are simultaneously applied to the outer periphery of the glass fiber and are subsequently cured. In the case where such a collective coating method is employed, since the power of a light source for irradiating ultraviolet rays is limited, it has been difficult for the primary coating layer to cure sufficiently, thereby making it problematic in that the manufacturing speed is rather lowered.
In order to avoid such a problem, for example, in the collective coating method such as that mentioned above, proposed is a technique in which a photo-polymerization initiator added to the secondary coating resin is relatively lowered (Japanese Patent Application Laid-Open Gazette No. Hei. 7-168069), or a technique in which the primary coating resin and secondary coating resin are cured with their respective curing wavelength bands shifted from each other (Japanese Patent Application Laid-Open Gazette No. Hei. 1-276105).
In the former of the conventional techniques mentioned above, however, when the amount of photo-polymerization initiator added to the secondary coating resin is lowered in order to cure the primary coating resin, the curing speed of the secondary coating decreases, whereby it may become problematic in that productivity deteriorates. In the latter technique, on the other hand, it is hard to obtain resins which clearly demarcate two curing wavelength bands, thus leaving a problem to cure two resin layers efficiently.
It is an object of the present invention to provide a coated optical fiber in which not only the secondary coating resin layer but also the primary coating resin layer can be photo-cured simultaneously and efficiently to a high degree of cure, without relatively lowering the photo-polymerization initiator added to the secondary coating resin or without shifting the curing wavelength bands of primary and secondary coating resins from each other; and a method of making the same.